Abstract : Exposures to organochlorine compounds have been suggested as risk factors for breast cancer because of their estrogen-like effect. The objective of this study is to determine if the high serum levels of various organochlorine compounds, detected 5-20 years before diagnosis, are risk factors for breast cancer, utilizing serums collected by the Janus Bank prior to diagnosis. Since we identified too few subjects in high risk occupations and there was concern about over matching, we chose to study employed women (including farmers wives) that also includes the high risk occupations. The original plan was to analyze the estrogen and anti-estrogen compounds on separate groups of cases and controls. However, protocol reviewers suggested that the study would be stronger if all compounds were measured on the same subjects. A selection algorithm has been developed, 150 cases and 150 matched controls have been selected and specimens will be delivered to the CDC on September 26th, 1995. Since only 1.1 ml of serum are available on each subject, the laboratory has been developing methods to assay both types of compounds on this amount of serum. Final tests are currently underway to see if the assays can measure these substances on 1.1 ml samples.